1. Field of the Invention
The present invention relates generally to a solid state imaging apparatus, and particularly to a solid state imaging apparatus of MOS type sensor.
2. Description of the Prior Art
The solid state imaging apparatus has advantages of small type, light weight, high reliability, low power consumption, etc., but on the other hand has such problem as having liability of blooming phenomenon (a phenomenon that when an imaging object is of high brightness, imaging information disappears due to over-flowing of excessive charge). In order to overcome the problem, various measures have been tried. Among them, there is a method that utilizes a PN junction as photoelectric transducing part, thereby making a layer including an impurity, which is along the PN junction on a side of substrate with respect to the PN junction. In that method, a depletion layer, which is formed so as to discharge the excessive charges into the substrate, can strongly suppress the blooming.
However, the above-mentioned method has a shortcoming that for exposures above a saturation level a fixed pattern noise is generated, and the process is too much sensitive to the impurity concentration-ununiformity, thereby making process control in mass-production much too complicated.
The above-mentioned shortcoming of the conventional measure of using the depletion part is described more in detail, taking a solid state imaging apparatus of a type called MOS type sensor, wherein MOS transistors are used for reading-out signal charges stored in photoelectric transducing part. In FIG. 1, which is a schematical plan view of a conventional MOS type sensor, a single crystalline wafer comprises a photoelectric transducer part 1 having a number of photoelectric transducer element 5, a vertical scanning circuit 2 and a horizontal shift registor 3, which are all disposed in a same impurity-doped layer 4 of the wafer.
Sectional structure of one unit of picture element of the photoelectric transducer element of FIG. 1 is shown in FIG. 2. In an n-conductivity type substrate 6, a shallow impurity-doped layer 7 of p-conductivity type is formed, and an n.sup.+ -conductivity type layer 8 is further formed in the shallow p-conductivity type layer. Also, a reading-out drain of n.sup.+ -conductivity type is formed in the shallow p-conductivity type layer 7 with a channel space 71 between the n.sup.+ -conductivity type layer 8. A gate 9 of a MOS transistor for reading-out signal charge is made by polycrystalline silicon electrode burred in an oxide film 13 at the part over the channel space 71. A vertical transmission wiring 11 of aluminum is provided to contact the n.sup.+ -reading-out drain 10. Across the n-conductivity type substrate 6 and the impurity-doped shallow p-conductivity type layer 7, an inverse potential V.sub.sub which is over a voltage to deplete the shallow impurity-doped p-type layer 7 is impressed on. A p.sup.+ -conductivity type channel stopper 12 is provided to surround each one set of the picture element comprising a photodiode part Ph-Di consisting of the substrate 6, the shallow layer 7 and n.sup.+ -conductivity layer 8 and an FET part consisting of an end part 81 of the n.sup.+ -conductivity type layer 8, the channel space 71 and the reading-out drain 10. The above-mentioned conventional configuration of the photoelectric transducing element 5 is described with reference to FIG. 3 which schematically shows potential profile along sectional plane A--A' of FIG. 2, illustrating function of suppressing blooming. Immediately after reading-out signal charge by gate 9 of the MOS transistor, the potential of the n.sup.+ -conductivity type layer 8 is set to a potential .phi..sub.AL of the vertical transmission wiring 11. The potential profile at that instant is shown by curve a. Then, as signal charge increases, potential of the n.sup.+ -conductivity type layer 8 decreases and the profile of the potential becomes as shown by curve b. Then, as the signal charge further increases, the potential of the n.sup.+ -conductivity type layer 8 comes almost to the lowest potential .phi..sub.T of the impurity-doped p-conductivity type layer 7, and the potential profile at this instant becomes as shown by curve c. Thereafter, when further signal charge flows in, thereafter excessive signal charge is exhausted into the substrate 6 by means of electric field, thereby blooming effect is suppressed. Since the above-mentioned configuration makes excessive signal charge from the n.sup.+ -conductivity type layer into the conductivity type substrate 6 under the condition of the electric field being impressed, the ability of suppressing of blooming is satisfactory. Level of the lowest potential .phi..sub.T represents minimum potential of the signal charge which can be stored in the n-conductivity type layer. That is, the level of .phi..sub.T defines the maximum amount of signal charge to be stored in the n-conductivity type layer 8 in each set of picture element. When the level of .phi..sub.T is the same in all of the picture elements of the wafer there is no problem, but in actual apparatuses the levels of .phi..sub.T are not necessarily uniform among the picture elements, and hence are likely to produce fixed pattern noise for higher exposures than saturated exposures, since stored signal charges of the n-conductivity type layers 8 of the picture elements are not uniform throughout the wafer. The level of .phi..sub.T is closely influenced by concentration of the impurity-doped p-conductivity type layer 7, which concentration is usually determined by compensation of boron ions or the like implanted into the n-conductivity type substrate 6 on the impurity of opposite conductivity type in the n-conductivity type substrate 6. That is, the impurity concentration of the impurity layer 7 is determined by a difference between the implanted boron concentration and the original concentration of the n-conductivity type substrate 6. While concentration of ions induced by the ion-implantation has good uniformity, impurity concentration of the n-conductivity type substrate 6 of the single crystal made by usual known CZ process or FZ process necessarily has ununiformity of impurity concentration in radial direction due to an ununiformity induced during crystal growth. Accordingly, the resultant impurity concentration of the impurity-doped p-conductivity type layer 7 necessarily reflects concentration ununiformity of the impurity of the substrate. Accordingly, photodiode parts Ph-Di of picture elements throughout the wafer necessarily have ununiformity of saturation level, which causes fixed pattern noise of co-centric pattern, and this pattern noise is problematic in lowering picture quality.